Transport Layer Security (TLS) is a cryptographic protocol designed to provide communications security over a computer network. The protocol is widely used in applications such as email, instant messaging, and voice over IP, but its use in securing HTTPS remains the most publicly visible.

The TLS protocol aims primarily to provide security, including privacy (confidentiality), integrity, and authenticity through the use of cryptography, such as the use of certificates, between two or more communicating computer applications. It runs in the presentation layer and is itself composed of two layers: the TLS record and the TLS handshake protocols.

The closely related Datagram Transport Layer Security (DTLS) is a communications protocol that provides security to datagram-based applications. In technical writing, references to "(D)TLS" are often seen when it applies to both versions.^[1]

TLS is a proposed Internet Engineering Task Force (IETF) standard, first defined in 1999, and the current version is TLS 1.3, defined in August 2018. TLS builds on the now-deprecated SSL (Secure Sockets Layer) specifications (1994, 1995, 1996) developed by Netscape Communications for adding the HTTPS protocol to their Netscape Navigator web browser.

Description

Client-server applications use the TLS protocol to communicate across a network in a way designed to prevent eavesdropping and tampering.

Since applications can communicate either with or without TLS (or SSL), it is necessary for the client to request that the server set up a TLS connection.^[2] One of the main ways of achieving this is to use a different port number for TLS connections. Port 80 is typically used for unencrypted HTTP traffic while port 443 is the common port used for encrypted HTTPS traffic. Another mechanism is to make a protocol-specific STARTTLS request to the server to switch the connection to TLS – for example, when using the mail and news protocols.

Once the client and server have agreed to use TLS, they negotiate a stateful connection by using a handshaking procedure (see § TLS handshake).^[3] The protocols use a handshake with an asymmetric cipher to establish not only cipher settings but also a session-specific shared key with which further communication is encrypted using a symmetric cipher. During this handshake, the client and server agree on various parameters used to establish the connection's security:

• The handshake begins when a client connects to a TLS-enabled server requesting a secure connection and the client presents a list of supported cipher suites (ciphers and hash functions).
• From this list, the server picks a cipher and hash function that it also supports and notifies the client of the decision.
• The server usually then provides identification in the form of a digital certificate. The certificate contains the server name, the trusted certificate authority (CA) that vouches for the authenticity of the certificate, and the server's public encryption key.
• The client confirms the validity of the certificate before proceeding.
• To generate the session keys used for the secure connection, the client either:
  • encrypts a random number (PreMasterSecret) with the server's public key and sends the result to the server (which only the server should be able to decrypt with its private key); both parties then use the random number to generate a unique session key for subsequent encryption and decryption of data during the session, or
  • uses Diffie–Hellman key exchange (or its variant elliptic-curve DH) to securely generate a random and unique session key for encryption and decryption that has the additional property of forward secrecy: if the server's private key is disclosed in future, it cannot be used to decrypt the current session, even if the session is intercepted and recorded by a third party.

This concludes the handshake and begins the secured connection, which is encrypted and decrypted with the session key until the connection closes. If any one of the above steps fails, then the TLS handshake fails and the connection is not created.

TLS and SSL do not fit neatly into any single layer of the OSI model or the TCP/IP model.^[4][5] TLS runs "on top of some reliable transport protocol (e.g., TCP),"^[6]: §1 which would imply that it is above the transport layer. It serves encryption to higher layers, which is normally the function of the presentation layer. However, applications generally use TLS as if it were a transport layer,^[4][5] even though applications using TLS must actively control initiating TLS handshakes and handling of exchanged authentication certificates.^[6]: §1

When secured by TLS, connections between a client (e.g., a web browser) and a server (e.g., wikipedia.org) will have all of the following properties:^[6]: §1

• The connection is private (or has confidentiality) because a symmetric-key algorithm is used to encrypt the data transmitted. The keys for this symmetric encryption are generated uniquely for each connection and are based on a shared secret that was negotiated at the start of the session. The server and client negotiate the details of which encryption algorithm and cryptographic keys to use before the first byte of data is transmitted (see below). The negotiation of a shared secret is both secure (the negotiated secret is unavailable to eavesdroppers and cannot be obtained, even by an attacker who places themselves in the middle of the connection) and reliable (no attacker can modify the communications during the negotiation without being detected).
• The identity of the communicating parties can be authenticated using public-key cryptography. This authentication is required for the server and optional for the client.
• The connection is reliable (or has integrity) because each message transmitted includes a message integrity check using a message authentication code to prevent undetected loss or alteration of the data during transmission.

TLS supports many different methods for exchanging keys, encrypting data, and authenticating message integrity. As a result, secure configuration of TLS involves many configurable parameters, and not all choices provide all of the privacy-related properties described in the list above (see the tables below § Key exchange, § Cipher security, and § Data integrity).

Attempts have been made to subvert aspects of the communications security that TLS seeks to provide, and the protocol has been revised several times to address these security threats. Developers of web browsers have repeatedly revised their products to defend against potential security weaknesses after these were discovered (see TLS/SSL support history of web browsers).

Datagram Transport Layer Security

Datagram Transport Layer Security, abbreviated DTLS, is a related communications protocol providing security to datagram-based applications by allowing them to communicate in a way designed^[7][8] to prevent eavesdropping, tampering, or message forgery. The DTLS protocol is based on the stream-oriented Transport Layer Security (TLS) protocol and is intended to provide similar security guarantees. However, unlike TLS, it can be used with most datagram oriented protocols including User Datagram Protocol (UDP), Datagram Congestion Control Protocol (DCCP), Control And Provisioning of Wireless Access Points (CAPWAP), Stream Control Transmission Protocol (SCTP) encapsulation, and Secure Real-time Transport Protocol (SRTP).

As the DTLS protocol datagram preserves the semantics of the underlying transport—the application it does not suffer from the delays associated with stream protocols, however the application has to deal with packet reordering, loss of datagram and data larger than the size of a datagram network packet. Because DTLS uses UDP or SCTP rather than TCP, it avoids the "TCP meltdown problem",^[9][10] when being used to create a VPN tunnel.

The original 2006 release of DTLS version 1.0 was not a standalone document. It was given as a series of deltas to TLS 1.1.^[11] Similarly the follow-up 2012 release of DTLS is a delta to TLS 1.2. It was given the version number of DTLS 1.2 to match its TLS version. Lastly, the 2022 DTLS 1.3 is a delta to TLS 1.3. Like the two previous versions, DTLS 1.3 is intended to provide "equivalent security guarantees with the exception of order protection/non-replayability".^[12]

Many VPN clients including Cisco AnyConnect^[13] & InterCloud Fabric,^[14] OpenConnect,^[15] ZScaler tunnel,^[16] F5 Networks Edge VPN Client,^[17] and Citrix Systems NetScaler^[18] use DTLS to secure UDP traffic. In addition all modern web browsers support DTLS-SRTP^[19] for WebRTC.

History and development

Secure Data Network System

The Transport Layer Security Protocol (TLS), together with several other basic network security platforms, was developed through a joint initiative begun in August 1986, among the National Security Agency, the National Bureau of Standards, the Defense Communications Agency, and twelve communications and computer corporations who initiated a special project called the Secure Data Network System (SDNS).^[26] The program was described in September 1987 at the 10th National Computer Security Conference in an extensive set of published papers. The innovative research program focused on designing the next generation of secure computer communications network and product specifications to be implemented for applications on public and private internets. It was intended to complement the rapidly emerging new OSI internet standards moving forward both in the U.S. government's GOSIP Profiles and in the huge ITU-ISO JTC1 internet effort internationally. Originally known as the SP4 protocol, it was renamed TLS and subsequently published in 1995 as international standard ITU-T X.274|ISO/IEC 10736:1995.

Secure Network Programming (SNP)

Early research efforts towards transport layer security included the Secure Network Programming (SNP) application programming interface (API), which in 1993 explored the approach of having a secure transport layer API closely resembling Berkeley sockets, to facilitate retrofitting pre-existing network applications with security measures. SNP was published and presented in the 1994 USENIX Summer Technical Conference.^[27][28] The SNP project was funded by a grant from NSA to Professor Simon Lam at UT-Austin in 1991.^[29] Secure Network Programming won the 2004 ACM Software System Award.^[30][31] Simon Lam was inducted into the Internet Hall of Fame for "inventing secure sockets and implementing the first secure sockets layer, named SNP, in 1993."^[32][33]

SSL 1.0, 2.0, and 3.0

Netscape developed the original SSL protocols, and Taher Elgamal, chief scientist at Netscape Communications from 1995 to 1998, has been described as the "father of SSL".^{[34][35][36][37]} SSL version 1.0 was never publicly released because of serious security flaws in the protocol. Version 2.0, after being released in February 1995 was quickly found to contain a number of security and usability flaws. It used the same cryptographic keys for message authentication and encryption. It had a weak MAC construction that used the MD5 hash function with a secret prefix, making it vulnerable to length extension attacks. It also provided no protection for either the opening handshake or an explicit message close, both of which meant man-in-the-middle attacks could go undetected. Moreover, SSL 2.0 assumed a single service and a fixed domain certificate, conflicting with the widely used feature of virtual hosting in Web servers, so most websites were effectively impaired from using SSL.

These flaws necessitated the complete redesign of the protocol to SSL version 3.0.^[38][36] Released in 1996, it was produced by Paul Kocher working with Netscape engineers Phil Karlton and Alan Freier, with a reference implementation by Christopher Allen and Tim Dierks of Certicom. Newer versions of SSL/TLS are based on SSL 3.0. The 1996 draft of SSL 3.0 was published by IETF as a historical document in RFC 6101.

SSL 2.0 was deprecated in 2011 by RFC 6176. In 2014, SSL 3.0 was found to be vulnerable to the POODLE attack that affects all block ciphers in SSL; RC4, the only non-block cipher supported by SSL 3.0, is also feasibly broken as used in SSL 3.0.^[39] SSL 3.0 was deprecated in June 2015 by RFC 7568.

TLS 1.0

TLS 1.0 was first defined in RFC 2246 in January 1999 as an upgrade of SSL Version 3.0, and written by Christopher Allen and Tim Dierks of Certicom. As stated in the RFC, "the differences between this protocol and SSL 3.0 are not dramatic, but they are significant enough to preclude interoperability between TLS 1.0 and SSL 3.0". Tim Dierks later wrote that these changes, and the renaming from "SSL" to "TLS", were a face-saving gesture to Microsoft, "so it wouldn't look the IETF was just rubberstamping Netscape's protocol".^[40]

The PCI Council suggested that organizations migrate from TLS 1.0 to TLS 1.1 or higher before June 30, 2018.^[41][42] In October 2018, Apple, Google, Microsoft, and Mozilla jointly announced they would deprecate TLS 1.0 and 1.1 in March 2020.^[20] TLS 1.0 and 1.1 were formally deprecated in RFC 8996 in March 2021.

TLS 1.1

TLS 1.1 was defined in RFC 4346 in April 2006.^[43] It is an update from TLS version 1.0. Significant differences in this version include:

• Added protection against cipher-block chaining (CBC) attacks.
  • The implicit initialization vector (IV) was replaced with an explicit IV.
  • Change in handling of padding errors.
• Support for IANA registration of parameters.^[44]

Support for TLS versions 1.0 and 1.1 was widely deprecated by web sites around 2020, disabling access to Firefox versions before 24 and Chromium-based browsers before 29.^[45][46]

TLS 1.2

TLS 1.2 was defined in RFC 5246 in August 2008.^[23] It is based on the earlier TLS 1.1 specification. Major differences include:

• The MD5 and SHA-1 combination in the pseudorandom function (PRF) was replaced with SHA-256, with an option to use cipher suite specified PRFs.
• The MD5 and SHA-1 combination in the finished message hash was replaced with SHA-256, with an option to use cipher suite specific hash algorithms. However, the size of the hash in the finished message must still be at least 96 bits.^{[23]: §7.4.9}
• The MD5 and SHA-1 combination in the digitally signed element was replaced with a single hash negotiated during handshake, which defaults to SHA-1.
• Enhancement in the client's and server's ability to specify which hashes and signature algorithms they accept.
• Expansion of support for authenticated encryption ciphers, used mainly for Galois/Counter Mode (GCM) and CCM mode of Advanced Encryption Standard (AES) encryption.
• TLS Extensions definition and AES cipher suites were added.^[44]

All TLS versions were further refined in RFC 6176 in March 2011, removing their backward compatibility with SSL such that TLS sessions never negotiate the use of Secure Sockets Layer (SSL) version 2.0. There is currently no formal date for TLS 1.2 to be deprecated. The specifications for TLS 1.2 became redefined as well by the Standards Track Document RFC 8446 to keep it as secure as possible plus it is to be seen as a failover protocol now, meant only to be negotiated with Clients which are unable to talk over TLS 1.3 (The original RFC 5246 definition for TLS 1.2 is since then obsolete!).

TLS 1.3

TLS 1.3 was defined in RFC 8446 in August 2018.^[6] It is based on the earlier TLS 1.2 specification. Major differences from TLS 1.2 include:^[47]

• Separating key agreement and authentication algorithms from the cipher suites^{[44][6]: §11}
• Removing support for weak and less-used named elliptic curves
• Removing support for MD5 and SHA-224 cryptographic hash functions
• Requiring digital signatures even when a previous configuration is used
• Integrating HKDF and the semi-ephemeral DH proposal
• Replacing resumption with PSK and tickets
• Supporting 1-RTT handshakes and initial support for 0-RTT
• Mandating perfect forward secrecy, by means of using ephemeral keys during the (EC)DH key agreement
• Dropping support for many insecure or obsolete features including compression, renegotiation, non-AEAD ciphers, null ciphers,^[48] non-PFS key exchange (among which are static RSA and static DH key exchanges), custom DHE groups, EC point format negotiation, Change Cipher Spec protocol, Hello message UNIX time, and the length field AD input to AEAD ciphers
• Prohibiting SSL or RC4 negotiation for backwards compatibility
• Integrating use of session hash
• Deprecating use of the record layer version number and freezing the number for improved backwards compatibility
• Moving some security-related algorithm details from an appendix to the specification and relegating ClientKeyShare to an appendix
• Adding the ChaCha20 stream cipher with the Poly1305 message authentication code
• Adding the Ed25519 and Ed448 digital signature algorithms
• Adding the x25519 and x448 key exchange protocols
• Adding support for sending multiple OCSP responses
• Encrypting all handshake messages after the ServerHello

Network Security Services (NSS), the cryptography library developed by Mozilla and used by its web browser Firefox, enabled TLS 1.3 by default in February 2017.^[49] TLS 1.3 support was subsequently added — but due to compatibility issues for a small number of users, not automatically enabled^[50] — to Firefox 52.0, which was released in March 2017. TLS 1.3 was enabled by default in May 2018 with the release of Firefox 60.0.^[51]

Google Chrome set TLS 1.3 as the default version for a short time in 2017. It then removed it as the default, due to incompatible middleboxes such as Blue Coat web proxies.^[52] Zdroj:https://en.wikipedia.org?pojem=SSL_encryption
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